I can't recall if Joomla (and the non open source likes) do file history/revisioning of documents. That's one plus that sharepoint has over some of the other free CMS's. If you can get that functionality out of a shared folder, I'd agree with you that it's basically the same. Sharepoint will also integrate with your active directory (I believe) and also exchange if you're already a full microsoft company. This would allow you to centrally manage users and permissions. I'd only argue that as a bonus if you have several hundred people using this tool.
With Sharepoint, you can designate a "section" that will have it's own document repositories, forums, blogs and many other tools that can be managed independently of other sections. This will allow you to have multiple CMS's within one server. The default layout looks like garbage the last I played with it, so unless you want to spend time designing a better look and feel, you'll be stuck with a crappy looking layout.
You could probably download a demo of sharepoint and throw it on a VM to play with it. I know it's not the simplest thing to do, but it might give you a better impression than I'm describing.
The bottom line is that we had push for it here and I wasn't excited for it after trying a demo. We ended up using a product called eRoom from a company we now own known as Documentum. I became frustrated with the sharepoint demo during my exposure to it. Setting it up and managing it seemed like a PITA.
If you are at all familiar with Microsoft's concept of "Web Parts", and you are comfortable developing them, you can add on to sharepoint fairly easily. You can make custom controls and features specific to your departments and add them right in. We're using web parts here at work and they seem to work fairly well.
